1. Field of the Invention
The present invention relates to a threshold matrix which is compared with a grayscale original image in generating a halftone dot image representing the original image.
2. Description of the Background Art
In many cases where printing data or printing plate data are made from grayscale images, creation of halftone dots, as one of halftoning method, is used. Among well-known types of dot shape are square, chain, elliptical one, round one or the like. In a generally-used AM (Amplitude Modulated) halftoning, grayscale representation is made by changing the size of dots, instead of changing the number of dots constituting a halftone dot image. But since the dots are regularly arranged, respective dots connect to adjacent dots almost simultaneous in a specified gray level (a gray level in specified tone value), and change of gray levels seems to be discontinuous even if intended change of gray levels is continuous. This is so-called tone jump. Also, there is a problem that there arises moire in a halftone dot image generated from a periodic image.
Japanese Patent Publication No. 3427026 (Document 1) discloses a method where after the centers of halftone cells are arranged irregularly, connecting of dots is performed at random and U.S. Pat. No. 5,859,955 (Document 2) discloses a method where a plurality of highlight-side halftone cell centers and a plurality of shadow-side halftone cell centers are arranged while being stochastically distributed. In the cases of using these methods, occurrence of moire is suppressed, connecting of dots occurs in a wide range of gray levels (for example, a range covering from about 20% to 30% of all gray levels around middle gray level) depending on growing method of halftone dots, and therefore the tone jump is reduced.
A technique disclosed in paragraph 0021 of Japanese Patent Publication No. 3481423 (Document 3) is that a smoothing operation is performed on a tint image by using a filter whose size is 4.25 times an average distance between the centers of dots in halftone dot areas, with respect to each of row and column directions.
In the cases of utilizing the methods of Documents 1 and 2, however, large clusters which are formed by connecting adjacent dots and small clusters which are isolated dots are mixed irregularly in a range of gray levels where connecting of dots occurs, and this causes graininess in a generated halftone dot image.